Over its long history, the game of golf has progressively evolved. This evolution has been especially pronounced in the equipment used to play golf. With the development of modern golf clubs, a number of variables and factors have been identified and refined to allow sets of clubs to be tailored for the respective golfers who use them. For example, some aspects of a club's “feel” as used during play have been identified and quantified sufficiently to allow those aspects to be controlled and even adjusted, either during manufacture of the club, at point of sale, or even later after the club has been used for actual play. Examples of these aspects of feel include size, shape, mass, and material of the clubhead, distribution of mass in the clubhead and shaft, clubhead material, shaft material, geometry and composition of the strike face, center of gravity and coefficient of restitution of the clubhead, etc.
Another factor is “swing-weight,” which is a measure of how the mass of the club feels as the club is being swung for hitting a golf ball. Generally, swing-weight reflects how mass is distributed in the club, as reflected by the club's perceived resistance to being moved during a “swing.” More specifically, the swing-weight is a measurement of a golf club's mass about a pivot point established at a specified distance from the grip-end of the club. Even more specifically, the swing-weight of a club is a measure of the club's moment of inertia about a point located 14 inches from the grip-end. Swing-weight can also be regarded as a measurement of a club's “balance,” i.e., the degree to which the club's mass balances toward the clubhead. A first club having a balance point located nearer the clubhead than a second club will generally feel heavier when swung than will the second club. Thus, key factors of swing-weight are shaft length and clubhead mass, with lesser contributions being made by other components and configurational details of the club.
Golfers have been at least subjectively aware of swing-weight for a long time, and many realize the importance of correlating the club's swing-weight to the natural speed at which a particular golfer can swing the club. Substantial mis-correlation in this regard can result in poor golfing performance. Golfers having a relaxed swing tend to do better with clubs having a heavier swing-weight. Golfers having a rapid swing, on the other hand, tend to do better with clubs having less swing-weight. Coordinating swing-weight to a golfer's swing allows the club to rotate around the pivot point of the golfer's body as rapidly as the golfer's body is being rotating while executing the swing.
The swing-weight scale was developed empirically in the 1920's. It currently has seven major indices: A, B, C, D, E, F, and G. Between each of these major indices are ten divisions (minor indices). Hence, a portion of the scale would be A0 1 2 3 4 5 6 7 8 9 B0 1 2 3 4 5 6 7 8 9 C . . . . (The actual numbers are not subscripts; but, they are so written here for clarity.) The G index includes the numeral 10, yielding a total of 73 “points.” Each swing-weight point is a respective combination of a major index and a minor index, and each point has a respective numerical value, depending upon the club. For example, golf clubs used by men usually have a swing-weight in the range of D0 to D5, whereas a woman's golf club may have a swing-weight of C5 to C7, wherein C denotes a lower swing-weight than D, and D3 denotes a greater swing-weight than D2. Swing-weights are usually tabulated for each of various values of shaft mass. Each table lists successive club lengths and corresponding swing-weight values based on clubhead mass.
The swing-weight scale was conceived at a time when golf-club shafts were usually made of wood, which is generally less flexible than many modern shaft materials. As a result, the swing-weight scale was developed under the assumption that the club's moment of inertia was usually about 14 inches from the grip-end of the shaft. Nowadays, shafts made of non-wood materials are usually more flexible than wooden shafts, and with most golfers these clubs tend to rotate during a swing about a point located closer than 14 inches from the butt-end. Nevertheless, the swing-weight scale based upon a 14-inch fulcrum has survived and is still used. Swing-weight is usually determined using a measurement device.
Swing-weight can be very sensitive to dimensional variations in a population of otherwise similar golf clubs. For example, during manufacture of a golf club, dimensions, material specifications, processes, configurational details, and the like of each component of the club normally have respective tolerances. Shaft lengths also exhibit a length tolerance even if they all have nominally the same length. During a production run in which a lot of clubs having a particular design are manufactured, tolerance stack-up naturally results in the clubs having respective swing-weights that vary slightly from one club to the next in the lot, for example from D0 to D5. Swing-weight is also affected by tolerances or changes in the grip. For example, increasing the size of the grip generally reduces the swing-weight of a club.
Swing-weight does not necessarily correlate with club mass. Stiffer shafts tend to have “lighter” feel, and more flexible shafts tend to have “heavier” feel. For example, graphite shafts have less mass but are more flexible than metal shafts. Changing a shaft from metal to graphite, for example, generally reduces the mass of the club while generally increasing its swing-weight.
Changing the “balance” of a club, and hence the club's swing-weight, is conventionally achieved by changing the location of a unit of discretionary mass on the club and/or by changing the actual mass of the unit. For example, consider three otherwise identical clubs in which only the location of the unit of discretionary mass is changed from one club to the next. The magnitude of heaviness “feel” of a club will vary depending upon the location of the unit of discretionary mass, even though all have exactly the same total mass. This change is particularly evident if the first club has the unit of mass located on the clubhead, the second club has the unit of mass located on the shaft, and the third club has the unit of mass located on the grip. The third club will have a lower swing-weight, and thus feel lighter when swung, than either the first club or the second club.
Swing-weight is often an issue at the time of sale of a set of clubs, particularly for experienced golfers. Not only is the ideal swing-weight for the golfer usually determined at this time, but also it is desirable that substantially all the clubs in the set have the same or closely similar swing-weight so that the golfer need not change his swing each time he uses a different club from the set.
A conventional hosel-plug scheme is shown in FIG. 1, depicting an iron-type clubhead 10. The clubhead 10 includes a sole 12, a heel 14, a toe 16, and a hosel 18. Also shown is a plug 20 that is inserted into the hosel 18 for changing the swing-weight of the club. The plugs 20 are made of various materials to provide similarly sized plugs having different respective weights that can be individually selected. In FIG. 1, as mass is added to or removed from the hosel 18 (e.g., adding a heavier plug, adding a lighter plug, or removing the plug), the mass properties of the clubhead (more specifically the position of the clubhead's center of gravity (CG)) typically change. Example data are listed in Table 1, below, for two otherwise similar irons having different hosel configurations A and B:
TABLE 1ClubNom.5 g10 gTot.Nom.5 g10 gTot.HeadCGXCGXCGX*CGX*ZupZupZup*Zup*A1.72.63.51.819.619.820.10.5B3.34.14.91.619.219.419.50.3wherein “Nom.” means nominal (no added weight), * indicates data obtained for the club including the 10-gram hosel weight, “Tot.” is the 10-g data less the corresponding nominal data, CGX refers to center-of-gravity (CG) position (mm) along the heel-to-toe axis (X-axis), and Zup refers to CG position (mm) along the Z-axis (vertical axis). Thus, by inserting a 10-g plug 20 into the hosel 18, the CG of the clubhead 10 shifts in position along the X-axis exactly 1.8 mm toward the hosel 18 and exactly 0.5 mm in the vertical direction (Z-direction). This positional shifting of the CG can have a substantial effect on the performance of the club during play, influencing not only its left-right distribution of mass but also the trajectory of a ball struck by the club.
Beyond the influence of ball flight, alteration of CG location can also influence the feel of a golf club, such as an iron. Striking a ball on the strike face at a location displaced from the CG has an adverse effect on the feel generated by that shot. In contrast, strikes occurring at the CG produce quieter, purer sounds than off-center strikes. Better players will attest to this and often find the CG of their clubs changed over time through use. This can be seen in many instances by examination of the round wear pattern on a club face of a well-used iron. On the clubs of a highly skilled player, the round pattern is usually slightly toward the heel of the clubhead and is in approximate alignment with the position of the CG. Hence, it is desirable to preserve the location of the CG of a clubhead even as other changes are made to the club.
Another conventional manner of changing swing-weight involves inserting a longitudinally extended insert into the shaft near the tip-end. Unfortunately the insert, usually called a “nail,” is prone to rattling or other vibration inside the shaft during use of the club, especially whenever the club is used in play. The nail can be made of any of various materials to provide different respective masses, typically aluminum, brass, and steel.
Yet another conventional manner of changing swing-weight is discussed in U.S. Pat. No. 7,871,339 to Sanchez. A balance weight is selected from multiple balance weights and mounted in a weight cavity formed in the clubhead. The balance weight is then covered with a “badge” cover that is bonded to the clubhead. One difficulty with this system is that the badge, after being bonded to the weight cavity, ordinarily cannot be removed without damaging or destroying at least the badge and clubhead.